The present invention relates to an optical writing head, in particular to an optical writing head in which the deviation of a line of light spots due to manufacturing dispersion of a rod-lens array. The present invention further relates to a method of correcting the deviation of a line of light spots.
A writing head of an optical printer, i.e. an optical writing head is a light source for exposing a photosensitive drum and comprises a line of light-emitting points consisting of a light-emitting element array. The structure of an optical printer including an optical writing head is shown in FIG. 1. An optically conductive material (photosensitive material) such as amorphous Si is provided on the surface of a cylindrical drum 2, which is rotated at the printing speed. The surface of the photosensitive material is uniformly charged with an electrostatic charger 4. Then, light corresponding to a dot image being printed with an optical writing head 6 is projected onto the surface of the photosensitive material to neutralize the charge on the area to which the light is projected. Next, a developer 8 deposits the toner on the photosensitive material surface in accordance with the charged pattern on the photosensitive material surface. The transfer unit 10 transfers the toner on a paper sheet 14 fed from a cassette 12. The toner on the paper sheet is thermally fixed by the heat applied by a fixer 16, and the paper is sent to a stacker 18. Upon completion of transfer, on the other hand, the charge on the drum is neutralized over the entire surface with an erasing lamp 20, and the remaining toner is removed by a cleaner 22.
The construction of the optical print head 6 is shown in FIG. 2. This optical print head comprises a light-emitting element array 24 and a rod-lens array 26, and the lens is adapted so as to focus on the photosensitive drum 2. The rod-lens array be composed of alternately stacked rod lenses, for example.
The inventors of the present invention have interested in a three-terminal light-emitting thyristor having a PNPN-structure as an element of the light-emitting element array, and have already filed several patent applications (see Japanese Patent Publication Nos. 1-238962, 2-14584, 2-92650, and 2-92651.) These patent publications have disclosed that a self-scanning function for light-emitting elements may be implemented, and further have disclosed that such self-scanning light-emitting element array has a simple and compact structure for the light source of a printer, and has smaller array pitch of thyristors.
The inventors have further provided a self-scanning light-emitting element array having such structure that an array of light-emitting thyristors having transfer function is separated from an array of light-emitting thyristors having writable function (see Japanese Patent Publication No. 2-263668.)
Referring to FIG. 3, there is shown an equivalent circuit diagram of a fundamental structure of this self-scanning light-emitting element array (two-phase driving and cathode common type). According to this structure, the light-emitting element array comprises transfer elements T1, T2, T3 . . . and writable light-emitting elements L1, L2, L3 . . . , these elements consisting of three-terminal light-emitting thyristors. The structure of the portion of an array of transfer elements includes diode D1, D2, D3 . . . as means for electrically connecting the gate electrodes of the neighboring transfer elements to each other. VGK is a power supply (normally 5 volts), and is connected to all of the gate electrodes G1, G2, G3 . . . of the transfer elements via a load resistor RL, respectively. Respective gate electrodes G1, G2, G3 . . . are correspondingly connected to the gate electrodes of the writable light-emitting elements L1, L2, L3 . . . . A start pulse xc3x8S is applied to the gate electrode of the transfer element T1, transfer clock pulses xc3x81 and xc3x82 are alternately applied to all of the anode electrodes of the transfer elements, and a write signal xc3x8I is applied to all of the anode electrodes of the light-emitting elements.
The operation of this self-scanning light-emitting device will now be described briefly. Assume that as the transfer clock xc3x81 is driven to a high level, the transfer element T2 is now turned on. At this time, the voltage of the gate electrode G2 is dropped to a level near zero volt from 5 volts. The effect of this voltage drop is transferred to the gate electrodes G3 via the diode D2 to cause the voltage of the gate electrode G3 to set about 1 volt which is a forward rise voltage (equal to the diffusion potential) of the diode D2. On the other hand, the diode D1 is reverse-biased so that the potential is not conducted to the gate G1, then the potential of the gate electrode G1 remaining at 5 volts. The turn on voltage of the light-emitting thyristor is approximated to a gate electrode potential+a diffusion potential of PN junction (about 1 volt.) Therefore, if a high level of a next transfer clock pulse xc3x82 is set to the voltage larger than about 2 volts (which is required to turn-on the transfer element T3) and smaller than about 4 volts (which is required to turn on the transfer element T5), then only the transfer element T3 is turned on and other transfer elements remain off-state, respectively. As a result of which, on-state is transferred from T2 to T3. In this manner, on-state of transfer elements are sequentially transferred by means of two-phase clock pulses.
The start pulse xc3x8S works for starting the transfer operation described above. When the start pulse xc3x8S is driven to a low level (about 0 volt) and the transfer clock pulse xc3x82 is driven to a high level (about 2-4 volts) at the same time, the transfer element T1 is turned on. Just after that, the start pulse xc3x8S is returned to a high level.
Assuming that the transfer element T2 is in the on-state, the voltage of the gate electrode G2 is lowered to almost zero volt. Consequently, if the voltage of the write signal xc3x8I is higher than the diffusion potential (about 1 volt) of the PN junction, the light-emitting element L2 may be turned into an on-state (a light-emitting state.)
On the other hand, the voltage of the gate electrode G1 is about 5 volts, and the voltage of the gate electrode G3 is about 1 volt. Consequently, the write voltage of the light-emitting element L1 is about 6 volts, and the write voltage of the light-emitting element L3 is about 2 volts. It follows from this that the voltage of the write signal xc3x8I which can write into only the light-emitting element L2 is in a range of about 1-2 volts. When the light-emitting element L2 is turned on, that is, in the light-emitting state, the amount of light thereof is determined by the write signal xc3x8I. Accordingly, the light-emitting elements may emit light at any desired amount of light. In order to transfer on-state to the next element, it is necessary to first turn off the element in on-state by temporarily dropping the voltage of the write signal xc3x8I down to zero volts.
The self-scanning light-emitting element array described above may be fabricated by arranging a plurality of light-emitting element array chips in one line so as to have a desired number of light-emitting points.
In an optical writing head using such self-scanning light-emitting element array, there is a problem in that a line of light spots which are projected on the photosensitive drum is deviated from a straight line in a sub-scanning direction due to the manufacturing dispersion of a rod-lens array. FIG. 4 shows the condition where a line of light spots is deviated in a sub-scanning direction. The light 41 emitted from each light-emitting point 40 of a straight-line light-emitting element array 28 passes through a rod-lens array 26 constructed by alternately stacked rod-lenses 27. The light passed through the rod-lens array is projected onto the photosensitive drum (not shown) to form a line of light spots 42.
It is apparent from the figure that a line of light spots deviates in a sub-scanning direction to draw a slightly convex line. It is noted herein that the sub-scanning direction is a direction perpendicular to an axis of rotation of the drum. A main-scanning direction is that in parallel with the axis of rotation of the photosensitive drum.
An object of the present invention is to provide an optical writing head in which the deviation of a line of light spots on the photosensitive drum in a sub-scanning direction due to manufacturing dispersion of a rod-lens array is corrected.
Another object of the present invention is to provide a method of correcting a deviation of light spots due to manufacturing dispersion of a rod-lens array.
According to the first aspect of the present invention, an optical writing head is provided, which comprises:
a self-scanning light-emitting element array consisting of a plurality of chips, each chip including
a self-scanning transfer element array having such a structure that a plurality of three-terminal transfer elements each having a control electrode for controlling threshold voltage or current are arranged, the control electrodes of the transfer elements neighbored to each other are connected via first electrical means, a power supply line is connected to the control electrodes via second electrical means, clock lines are connected to one of two terminals other than the control electrodes of each of the transfer elements, and a start pulse line is connected to the control electrode of the transfer element arranged at an end; and a light-emitting element array having such a structure that a plurality of three-terminal light-emitting elements each having a control electrode for controlling threshold voltage or current are arranged, the control electrodes of the light-emitting element are correspondingly connected to the control electrodes of the transfer elements, and a line for applying a write signal connected to one of two terminals other than the control electrode of each of the light-emitting elements is provided, and
a rod-lens array for projecting light emitted from the self-scanning light-emitting element array onto a lightsensitive drum to form a line of light spots,
wherein the timing of a start pulse supplied to the start pulse line every chip is regulated to correct the deviation of the line of light spots due to manufacturing dispersion of the rod-lens.
According to the second aspect of the present invention, an optical writing head is provided, which comprises:
a self-scanning light-emitting element array consisting of a plurality of chips, each chip including
a self-scanning transfer element array having such a structure that a plurality of three-terminal transfer elements each having a control electrode for controlling threshold voltage or current are arranged, the control electrodes of the transfer elements neighbored to each other are connected via first electrical means, a power supply line is connected to the control electrodes via second electrical means, clock lines are connected to one of two terminals other than the control electrodes of each of the transfer elements, and a start pulse line is connected to the control electrode of the transfer element arranged at an end; and a light-emitting element array having such a structure that a plurality of three-terminal light-emitting elements each having a control electrode for controlling threshold voltage or current are arranged, the control electrodes of the light-emitting element are correspondingly connected to the control electrodes of the transfer elements, and a line for applying a write signal connected to one of two terminals other than the control electrode of each of the light-emitting elements is provided, and
a rod-lens array for projecting light emitted from the self-scanning light-emitting element array onto a lightsensitive drum to form a line of light spots,
wherein the plurality of chips are divided into blocks each consisting of a given number of chips, one start pulse line is provided every block, and the timing of a start pulse supplied to the one start pulse line is regulated every block to correct the deviation of the line of light spots due to manufacturing dispersion of the rod-lens.